Automotive vehicle body structures often include closure members, such as, doors, hoods, deck lids, tailgates, and the like that have complementary inner and outer panels that are joined at peripheral surfaces. Sometimes the manufacture of the body includes the attachment of a body panel like a vehicle top to a body frame structure. Where the panel members are both made of stamped metal alloys the pieces are often hemmed and welded at their edges. However, when one or both of the panel members are formed of a fiber reinforced polymer material, a bead of adhesive is applied to flange surfaces of one or both panels. The panels are positioned with overlying joining surfaces and pressed together and the assembly is heated to cure the adhesive bond. A continuing difficulty arises in that a surface deformation along the adhesive bond line is visible in outer (un-bonded) surfaces of the exterior panel. This bond-line read-out effect is usually viewed as a defect in the surface of the door or panel assembly when it is visible to the user of the vehicle. The avoidance or repair of such defects has been a continuing problem for automotive manufacturers.
Many polymeric vehicle body panels are made of sheet molding compound (SMC). SMC is a glass fiber reinforced thermosetting composition in which the polymer precursor material typically comprises styrene, an unsaturated polyester, filler, maturation agent, and catalyst. The reinforcing material comprises glass mat and chopped glass roving. The material is prepared in sheet form enclosed in sheets of polyethylene film. These sheets are laid in molds for forming thermoset body panels and other parts. Like panels may also be made from carbon composite materials which are thermosetting polymers reinforced with carbon fibers, often in the form of cloth layers.
Epoxy based adhesives are widely used for bonding SMC panels or other reinforced polymer panels. The epoxy precursor materials are often relatively low molecular weight addition polymers of bisphenol A and/or bisphenol F and epichlorohydrin. Amine-group terminated compounds may be used as catalysts. Urethane adhesives are also used in bonding reinforced polymer panels. These adhesives provide good bond strength in joining complementary panels or in attaching panels to metal frame members. But they have higher coefficients of thermal expansion than those of the polymeric or metal panels or frame members. Due to the difference in coefficient of thermal expansion, the adhesive and the polymer panels experience different degrees of expansion or shrinkage during thermal cycling for adhesive curing, paint baking, or other processing or environmental temperature cycling. The differential in thermal expansion/shrinkage of panel and the adhesive creates differential residual stresses at the joint that deforms the exterior panel yielding the visible bond-line.
Unsuccessful attempts have been made to eliminate the bond-line read-out by adding ten to forty percent by volume of glass and/or steel microspheres (of 75 to 150 micrometer diameters). The use of such fillers reduces the bond-line effect by reducing the difference in thermal expansion between the parts and adhesive. But the excessive filler content affects the strength of the adhesive bond. There remains a need to address the adhesive bonding of polymeric panels or other parts having surface appearance requirements.